Micro-wave amplifier utilizing the interaction between an electron beam and a plasma stream



30 96 KAZUO AYAKI 3,270,244

MICRO-WAVE AMPLIFIER UTILIZING THE INTERACTION BETWEEN AN ELECTRON BEAM AND A PLASMA STREAM Filed Jan. 23, 1964 757644 M 5755: 92A; f-fbfrm United States Patent 3,270,244 MICRO-WAVE AMPLIFIER UTILIZING THE INTER- ACTION BETWEEN AN ELECTRUN BEAM AND A PLASMA STREAM Kazuo Ayaki, Tokyo, Japan, assignor to Nippon Electric Company Limited, Tokyo, Japan Filed Jan. 23, 1964, Ser. No. 339,773 Claims priority, application Japan, Jan. 29, 1963, 38/ 3,976 6 Claims. (Cl. 315-39) The instant invention relates to amplifying devices and more particularly to a novel micro-wave amplifying device employing plasma principles.

The amplification of micro-waves by the interaction between an electron beam and a plasma through which the electron beam passes has heretofore been studied and several theoretical analyses and experiments have been reported regarding such studies. The mechanisms of amplification in such theories and experiments may be divided into two general classes:

The first class is that in which maximum gain is obtainable when the plasma frequency very nearly coincides with the frequency of the signal to be amplified; the other general classification is that in which an electrical charge wave, in a cylindrical plasma, for example, is provided, which electrical charge wave propagates through the plasma at a speed less than that of the speed of light in a mode similar to that of a surface wave or the like whereby amplification of the micro-waves is achieved by the interaction of such a wave and the electron beams when their speeds or velocities are made very nearly equal.

In the latter classification, the frequency at which maximum gain is obtainable does not generally coincide with the plasma frequency. Examples of experiments covering both of the general classifications set forth above and the equipments employed in the performance of these experiments are disclosed in the Proceedings of the IRE, volume 49, No. 12, December 1961, on pages 1906-1916.

In the mechanisms used for amplification which are described in the above article, the electron beam is modulated longitudinally [i.e. in the direction in which the electron beam is moving] and the longitudinal wave is then amplified by an interaction between the plasma and the electron beam.

In the instant invention, however, unlike any of the amplification mechanisms presently known, a uniform magnetic field is applied in the direction of travel of the electron beam so as to transversely modulate the beam, that is, in the direction perpendicular to the direction of travel of the electron beam, and subsequent thereto the electron beam is then passed through the plasma for amplification thereof.

In conventional plasma amplification apparatus of the longitudinal modulation type, in order to provide extremely high operating frequencies it is necessary to have the plasma frequency equal to or greater than the operating frequency of the system, thus necessitating the maintenance of extremely high plasma electron density. It has been shown, however, that it is extremely diflicult to produce a stable and quiet plasma having such high electron density.

In accordance with the present invention, there is provided an improved amplifier device in which the plasma frequency may be substantially lower than the operating frequency and hence substantially lower than the plasma frequencies of present day devices, thus eliminating the requirement of providing extremely high density plasma at such increased operating frequencies.

The instant invention is comprised of a air-tight envelope or housing having an electron beam source. The

electron beam passes through a first coupling means which is operated in such a manner as to transversely modulate the electron beam passing therethrough such that the modulation occurs in the direction substantially perpendicular to the direction of movement of the electron beam. The modulated electron beam is then. passed through the second portion of the envelope means constituting the plasma region. Plasma is generated by ionizing cesium with suitable heating means to produce the desired plasma. Other means for producing the desired plasma may be employed, the particular method set forth being only one exemplary embodiment. Another method of producing the plasma which may be employed is that of the hot cathode discharge of mercury.

The electron beam reacts with the plasma generated in the housing and transverse high frequency components contained in the electron beam are greatly amplified. The electron beam then moves out of the plasma region and into a region provided with second coupler means in which the high frequency energy is induced in the coupler means, which energy may then be suitably imposed upon the utilization circuit. By utilization of transverse modulation of the electron beam, the instant arrangement difiers markedly from prior art devices which employ longitudinal modulation means, thus requiring extremely high density plasma at high operating frequencies, which requirements are necessarily obviated by the arrangement of the instant invention.

It is therefore one object of the instant invention to provide novel micro-wave amplifying means for amplifying high frequency signals through the interaction of an electron beam with a plasma.

Still another object of the instant invention is to provide novel micro-wave amplifying means for greatly amplifying high frequency signals through the interaction of an electron beam with a plasma wherein novel means are provided for transversely amplifying the electron beam.

Still another object of the instant invention is to provide novel micro-wave amplifying means for greatly amplifying high frequency signals through the interaction of an electron beam with a plasma wherein novel means are provided for transversely amplifying the electron beam, thus avoiding the need for providing extremely high density plasma at high operating frequencies.

These and other objects of the instant invention will become apparent when reading the accompanying description and drawing in which the sole figure is a crosssectional view of a micro-wave amplifying device designed in accordance with the principles of the instant invention:

In the drawing there is shown a micro-wave amplifying device designed in accordance with the principles of the instant invention, which device 10 is comprised of a suitable housing or envelope 20 for housing an electron gun 11 at one end thereof. The leads 11a of electron gun 11 may be connected to any suitable voltage sources in a well known manner in order to appropriately power the electron gun 11. The electron gun 11 generates an electron beam 12 which moves from the left to the right in the device 10.

The electron beam 12 in moving away from electron gun 11 and toward the right passes through a coupling means 13 which imparts a transverse modulation to the electron beam 12. Coupling means 13 may be a Cuccia type coupler having suitable electrodes 13a and 1311 connected to appropriate a high frequency voltage source [not shown] for the purpose of transversely modulating the electron beam 12. The coupler means 13 comprises two plate-like electrodes which produce a high frequency electric field between them by virtue of the external signal applied thereto. This high frequency electric field 3 is perpendicular to the direction of travel of electron beam 12 so that when the electron beam passes therethrough the beam is modulated laterally by the high frequency electric field generated between the electrode plates.

The electron beam 12 which has then been suitably transversely modulated by coupler means 13 then enters the plasma region 14. The plasma region is located within the envelope 20 at a position intermediate the ends thereof and is filled with cesium vapor which is ionized by heating the tungsten heaters .15 and 15' which are provided with suitable terminals 15a15b and 15a-15b so as to pass an electric current through the tungsten heaters 15 and .15 to produce plasma in the tube. A longitudinal-type plasma amplifier device employing such a plasma generating process as is shown in the sole figure is described in detail in the Physical Review Letters, volume 6, No. 4, February '15, 196-1, on pages 163165. A detailed description of such a longitudinal type plasma amplifier is therefore omitted from the instant description for purposes of clarity, the above article being incorporated herein by reference thereto.

In the embodiment shown in the sole figure the method of generating cesium plasma by thermal ionization has been described as one method for producing the plasma. However, the invention is not limited to this particular method, but, for example, plasma resulting from the hot cathode discharge of mercury may also be used. Any other suitable plasma generating means may also be used without departing from the scope of the instant invention.

The electron beam 12 which has been transversely modulated by coupling means 13 passes into the plasma region 14 and interacts with the plasma in such a man ner that the transverse high frequency components contained in the electron beam are greatly amplified.

The modulated electron beam 1-2 which has so reacted with the plasma in region 14, then passes through a second coupling means 16 which also may be of the Cucci a type similar to the coupling means 13. Coupling means 16 is likewise provided with suitable electrodes 16a and 16b which may be connected to a utilization device [not shown]. The transversely modulated high frequency components contained in the electron beam produce a voltage in the coupling means 16, which voltages are then imposed upon the utilization device which may be coupled to coupler 16.

The electron beam 12 ultimately impinges upon suitable collector means 17 positioned near the extreme righthand end of envelope 20, which, it should be understood, is a gas-tight chamber. The plasma region 14 is further provided with a pair of circular discs 19 19 having suitable apertures 19a and 19a to permit the passage of electron beam 12 therethrough. Circular discs are provided to prevent the plasma from diffusing into the region of electron gun 11 and coupler 16 and collector 17. In the embodiment shown in the sole figure, Cuccia type couplers 13 and 16 have been employed for the input and output couplers. This selection is merely exemplary and any other type of couplers which are capable of transversely modulating an electron beam may be employed as an alternative to the arrangement shown. Magnetic means 21 are provided to produce a uniform magnetic field which is applied in the direction of travel of the electron beam.

The marked difference from the embodiment shown in the sole figure from conventional longitudinal type plasma amplifiers is that such conventional devices employ couplers such as helices or cavities which perform longitudinal modulation upon the electron beam, whereas the instant invention which employs couplers such as, for example, Cuccia couplers, acts to modulate the electron beam in the transverse direction. This essential difference causes the operation between the instant invention and conventional mechanisms to be markedly different.

The amplification characteristics for a transversely modulated electron beam as it passes through a plasma under a uniform magnetic field is given by the following equation:

This equation is derived from Poissons equation and further from the equations of motion and equations of continuity for the electron beam and the plasma electrons, respectively. This equation is a dispersion equation for the negative rotational component when the transverse component of the electron beam is resolved into its positive and negative rotational components [with the latter component being the clockwise rotational component when viewed in a direction of travel of electrons] and with a magnetic field applied in the direction of travel of the electrons. In the above equation, w is the operating angular frequency; W is the cyclotron angular frequency of the electrons; W and w are the plasma angular frequencies for the plasma electron density and the electron beam density, respectively; and v is the linear velocity of the electron beam and B is a propagation constant.

It is found from the above equation that there is a frequency region in which B becomes an imaginary number, with the amplification taking place in this region. It will be noted that the actual amplification characteristic will be slightly different from that which is expected from the above equation, since the equation corresponds to a condition in which no consideration has been given to the thermal motion of the plasma electrons, the collision be tween particles and the boundaries of the plasma regions. Nevertheless, the esential aspects of the microwave amplifier 10 are conveyed by the above equation.

It will appear from the above equation that an amplification gain may be obtained at the frequency of 2800 mc., if the magnetic field intensity is 1000' gauss and the plasma density is l0 /cm.

As can be seen from the foregoing description, the present invention is characterized by the fact that the electron beam undergoes transverse modulation and then is passed through a plasma where through interaction therewith the high frequency components are greatly amplified and in leaving the plasma region the high frequency energy in the beam is then transferred to a coupler provided on the output side. Couplers and plasma generating devices which may be employed in the instant invention may be of various types with the types described herein being merely exemplary. Other alternatives fall within the scope of the instant invention. The microwave amplifier described herein may also be constructed to operate as an oscillator by feeding the energy from the output coupler back to the input coupler in order to provide oscillating action. Various other uses for the microwave amplifier of the instant invention may be contemplated without departing from the spirit of the instant invention.

Although there has been described a preferred embodiment of this novel invention, many variations and modifications will now be apparent to those skilled in the art. Therefore, this invention is to be limited, not by the specific disclosure herein, but only by the appending claims.

What is claimed is:

1. Micro-wave amplifying means comprising air-tight housing means; electron gun means mounted interior of said housing means near one end thereof for generating an electron beam directed toward the opposite end of said housing means; fiirst means located adjacent said electron gun means for transversely modulating said electron beam; second means for generating plasma located intermediate said first means and said housing means opposite end; third means for generating a magnetic field in the same direction as said electron beam whereby said electron beam and said plasma interact with each other in said magnetic field causing high frequency components of said modulated electron beam to become amplified; fourth means located near said housing means second end and substanially surounding said electron beam for transversely demodulating said modulated electron beam and thereby inducing signals in said fourth means representing the high frequency components of said modulated electron beam.

2. Micro-wave amplifying means comprising air tight housing means; electron gun means mounted interior of said housing means near one end thereof for generating an electron beam directed toward the opposite end of said housing means; first means located adjacent said electron gun means for transversely modulating said electron beam; second means for generating plasma located intermediate said first means and said housing means opposite end; third means for generating a magnetic field in the same dimotion as said electron beam whereby said electron beam and said plasma interact with each other in said magnetic field causing high frequency components of said modulated electron beam to become amplified; fourth means located near said housing means second end and substantially surrounding said electron beam for transversely demodulating said modulated electron beam and thereby inducing signals in said fourth means representing the high frequency components of said modulated electron beam; said first means comprising means for generating high frequency electric fields substantial-1y perpendicular to the direction of travel of said electron.

3. Micro-Wave amplifying means comprising air-tight housing means; electron gun means mounted interior of said housing means near one end thereof for generating an electron beam directed toward the opposite end of said housing means; first means located adjacent said electron gun means for transversely modulating said electron beam; second means for generating plasma located intermediate said first means and said housing means 0pposite end; third means for generating a magnetic field in the same direction as said electron beam whereby said electron beam and said plasma interact with each other in said magnetic field causing high frequency components of said modulated electron beam to become amplified; fourth means located near said housing means second end and substantially surrounding said electron beam for transversely demodulating said modulated electron beam and thereby inducing signals in said fourth means representing the high frequency components of said modulated electron beam; said first means comprising means for generating high frequency electric fields substantially perpendicular to the direction of travel of said electron; said first means being Cuccia type coupler means.

4. Micro-wave amplifying means comprising air-tight housing means; electron gun means mounted interior of said housing means near one end thereof for generating an electron beam directed toward the opposite end of said housing means; first means located adjacent said electron gun means for transversely modulating said electron beam; second means for generating plasma located intermediate said first means and said housing means opposite end; third means for generating a magnetic field in the same direction as said electron beam whereby said electron beam and said :plasma interact with each other in said magnetic field causing high frequency components of said modulated electron beam to become amplified; fourth means located near said housing means second end and substantially surrounding said electron beam for transversely demodulating said modulated electron beam and thereby inducing signals in said fourth means representing the high frequency components of said modulated electron beam; said first means comprising means for generating high frequency electric fields substantially perpendicular to the direction of travel of said electron; said first means and said fourth means being Cuccia type coupler means.

5. Microwave amplifying means comprising air-tight housing means; electron gun means mounted interior of said housing means near one end thereof for generating an electron beam directed toward the opposite end of said housing means; first means located adjacent said electron gun means for transversely modulating said electron beam; second means for generating plasma located intermediate said first means and said housing means opposite end; third means for generating a magnetic field in the same direction as said electron beam whereby said electron beam and said plasma interact with each other in said magnetic field causing high frequency components of said modulated electron beam to become amplified; fourth means located near said housing means second end and substantially surrounding said electron beam for transversely demodulating said modulated electron beam and thereby inducing signals in said fourth means representing the high frequency components of said modulated electron beam; said plasma generating means comprising at least one heating means; an ionizable material in the region of said heating means for producing said plasma.

6. Micro-wave amplifying means comprising air-tight housing means; electron gun means mounted interior of said housing means near one end thereof for generating an electron beam directed toward the opposite end of said housing means; first means located adjacent said electron gun means for transversely modulating said electron beam; second means for generating plasma located intermediate said first means and said housing means opposite end; third means for generating a magnetic field in the same direction as said electron beam whereby said electron beam and said plasma interact with each other in said magnetic field causing high frequency components of said modulated electron beam to become amplified; fourth means located near said housing means second end and substantially surrounding said electron beam for transversely demodulating said modulated electron beam and thereby inducing signals in said fourth means representing the high frequency components of said modulated electron beam; said plasma generating means comprising at least one heating means; an ionizable material in the region of said heating means for producing said plasma; first and second barrier means positioned at opposite ends of said plasma generating means and defining the region of said plasma to prevent said plasma from diffusing the other regions of said housing means; both of said barrier means having apertures to permit the passage therethrough of said electron beam.

References Cited by the Examiner UNITED STATES PATENTS 2,817,045 12/1957 Goldstein 315--3.5 2,848,649 8/1958 Bryant 315-39 3,171,053 2/1965 Targ 315-3.5

JAMES W. LAWRENCE, Primary Examiner. GEORGE N. WESTBY, Examiner.

S. D. SCHLOSSER, Assistant Examiner. 

4. MICRO-WAVE AMPLIFYING MEANS COMPRISING AIR-TIGHT HOUSING MEANS; ELECTRON GUN MEANS MOUNTED INTERIOR OF SAID HOUSING MEANS NEAR ONE END THEREOF FOR GENERATING AN ELECTRON BEAM DIRECTED TOWARD THE OPPOSITE END OF SAID HOUSING MEANS; FIRST MEANS LOCATED ADJACENT SAID ELECTRON GUN MEANS FOR TRANSVERSELY MODULATING SAID ELECTRON BEAM; SECOND MEANS FOR GENERATING PLASMA LOCATED INTERMEDIATE SAID FIRST MEANS AND SAID HOUSING MEANS OPPOSITE END; THIRD MEANS FOR GENERATING A MAGNETIC FIELD IN THE SAME DIRECTION AS SAID ELECTRON BEAM WHEREBY SAID ELECTRON BEAM AND SAID PLASMA INTERACT WITH EACH OTHER IN SAID MAGNETIC FIELD CAUSING HIGH FREQUENCY COMPONENTS OF SAID MODULATED ELECTRON BEAM TO BECOME AMPLIFIED; FOURTH MEANS LOCATED NEAR SAID HOUSING MEANS SECOND END AND SUBSTANTIALLY SURROUNDING SAID ELECTRON BEAM FOR TRANSVERSELY DEMODULATING SAID MODULATED ELEC- 